Peculiarities of Interactions of Alloying Elements with Grain Boundaries and the Formation of Segregations in Al-Mg and Al-Zn Alloys

The interactions between alloying elements and the symmetric edge boundary Sigma 5{210}[001] in Al alloys has been studied by the methods of the theory of the electronic-density functional. It has been shown that, in Al alloys, mechanisms of interactions of Mg and Zn with the grain boundary have been qualitatively different, which causes the peculiar morphology of the grain-boundary segregations. In the case of Mg, the deformational mechanism of interaction with the grain boundary is predominant, which facilitates the formation of relatively broad segregations. At the same time, in the case of Zn, the electronic mechanism, which is determined by directed chemical bonding, mainly contributes to interactions with the grain boundaries. As a result, it is more energetically advantageous for Zn atoms to occupy an interstitial position at the grain boundary or in the thin layer close to the grain boundary. The obtained results qualitatively explain the peculiar morphology of segregations at the grain boundary, which have been experimentally observed in the Al-Mg and Al-Zn alloys.